1. Field of the Invention
The present invention generally relates to catalysts for converting light hydrocarbons (e.g., methane) to products containing carbon monoxide and hydrogen (synthesis gas). More particularly, the invention relates to macroporous monolithic self-supported active catalyst structures and to non-poisoning catalyst support structures having a high surface-area-to-volume ratio.
2. Description of Related Art
The catalytic partial oxidation of light hydrocarbons, such as C1-C5 hydrocarbons, to yield products containing a mixture of carbon monoxide and hydrogen (xe2x80x9csynthesis gasxe2x80x9d or xe2x80x9csyngasxe2x80x9d) is currently an area of intense interest and investigation. Much of the work in this field has been described in the literature. One focal point of this research is on methane, the main component of natural gas, as a starting material for the production of higher hydrocarbons and hydrocarbon liquids in order to improve the economics of natural gas use. This is due to the fact that there is a great deal of natural gas available in many areas of the world, and the world""s natural gas supply is predicted to outlast the world""s oil reserves by a significant margin. Most of the world""s natural gas supply is situated in areas that are geographically remote from population and industrial centers, however. The costs of compression, transportation, and storage make the commercial use of natural gas economically unattractive. At the present time, commercial production of synthesis gas by methane conversion primarily utilizes steam reforming processes, which result in a similar gas product mixture to that obtained by partial oxidation of methane. Conventional steam reforming processes are well described in the literature.
In catalytic partial oxidation processes the gaseous hydrocarbon feedstock is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 1.
xe2x80x83CH4+1/2O2⇄CO+2H2xe2x80x83xe2x80x83(1)
This product ratio is especially desirable for such downstream applications as the conversion of the syngas to methanol, or conversion to hydrocarbon products such as fuels boiling in the middle distillate range (e.g. kerosene and diesel fuel) and hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis. The partial oxidation of methane is an exothermic reaction and proceeds at a faster rate than the older steam reforming processes for producing syngas. Shorter catalyst contact times and reduced scale reactors to accomplish partial oxidation of a hydrocarbon feedstock are some of the improvements made possible by a catalytic partial oxidation process.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Typically, the best catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by typical catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification, however. For successful operation at commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities (GHSV), and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (xe2x80x9ccokexe2x80x9d) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort has been devoted to the development of highly active catalysts allowing commercial performance without coke formation.
A number of process regimes have been described in the literature for the production of syngas via catalyzed partial oxidation reactions. The noble metals, which typically serve as the best catalysts for the partial oxidation of methane, are scarce and expensive. The widely used, less expensive, nickel-based catalysts have the disadvantage of promoting coke formation on the catalyst during the reaction, which results in loss of catalytic activity. Moreover, in order to obtain acceptable levels of conversion of gaseous hydrocarbon feedstock to CO and H2 it is typically necessary to operate the reactor at a relatively low flow rate, or gas hourly space velocity, using a large quantity of catalyst.
U.S. Pat. No. 4,810,685 (assigned to Imperial Chemical Industries PLC) discloses certain steam reforming catalysts containing strong ceramic foams comprising at least 50% by weight, of oxides of Fe, Co, Ni, Cu, Va, Mo, W, Cr, Mn or Zn. EPO 303,438 (assigned to Davy McKee Corporation) describes certain syngas catalysts that provide surface area to volume ratio of 5 cm2/cm3 to about 40 cm2/cm3. For example, high surface area alumina is deposited on a honeycomb monolith of cordierite material to serve as a support upon which finely dispersed catalytic metal components such as Pt and Pd are distended.
U.S. Pat. No. 5,648,582 (assigned to Regents of the University of Minnesota) discloses a process for the catalytic partial oxidation of methane at space velocities of 800,000 to 12,000,000 hrxe2x88x921 on certain Rh, Ni and Pt catalysts supported by a ceramic monolith. The small catalyst bed used in this process is said to eliminate hot spots which are typical of relatively thick catalyst beds.
Short contact time, reduced scale catalytic partial oxidation processes for producing syngas from methane require highly active catalysts and support structures that can function for a long period of time under the required high temperature and high pressure operating conditions. Most supported catalysts with good activity and high porosity tend to break down rapidly on-stream, however. Another problem that is encountered in many methane oxidation processes employing supported catalysts is that the porous support xe2x80x9cpoisonsxe2x80x9d the active catalyst. The poison is a component that is not present on the active catalyst overlayer and which affects catalyst performance in some way. In many cases, the poison interferes with the performance of the catalyst. Chemical interactions between the oxides of the group VIII transition metals and the reactive components of the support lead to the formation of catalytically inactive binary oxide phases. The catalytic phase purity is disrupted, or poisoned. For example, nickel forms nickel aluminate over an (xcex1-aluminate support. Some catalyst structures employ diffusion barriers between the active catalyst and the support structure in an attempt to ameliorate this problem.
Others have described processes pertaining to the sintering of chromium oxide, but the resulting sintered chromium oxide products are not suitable structures for use as syngas generation catalysts. For example, T. Li, et al. (1999 J. Eur. Ceram. Soc. 19:399-405) discloses a method of sintering Cr2O3 in H2/H2O gas mixtures. M. Li, et al. (1998 China""s Refract. 7:11-13) describes a key technique of manufacture of dense chromium sesquioxide refractories. S. Hashimoto, et al. (1996 J. Ceram. Soc. 104:1121-1124) describes densely sintered, compacted MgO and Cr2O3 powders. M. Yoshinaka, et al. (1995 J. Am. Ceram. Soc.78:2271-3) describe a hot isostatic pressed sol-gel derived chromium (III) oxide. A. Harabi, et al. (1995 Br. Ceram. Trans. 94:97-102) describe densification and grain growth in sintered alumina-chromia powder mixtures. Loss of chromia was substantial in compacts with more than 10 wt % chromia. U. Balachandran et al. (1995 Nanostruct. Mater. 5:505-12) describe the synthesis, sintering and magnetic properties of a nanophase Cr2O3 composition. R. Waesche, et al. (1995 Ceram. Trans. 51:531-5) describe the sintering and characterization of certain gelcast alumina-chromia reaction bonded ceramics. T. Li, et al. (1995 Ceram. Trans. 51:231-5) describes a process for making reaction-bonded Cr2O3 ceramics.
What is needed are economical, yet highly active catalyst structures that permit short contact time and high flow rates on stream without causing excessive back pressure and without deteriorating under operational temperature and pressure conditions. Superior syngas catalysts, for example, need to possess macroporosity (to minimize back pressure), thermal heat resistance and mechanical strength for use at high volumetric flow rates. The catalyst should also be free of interfering chemical interactions with the support material.
Conventional catalysts or catalyst support structures, particularly those employed for catalyzing the conversion of light hydrocarbons (such as methane) to synthesis gas, do not include free standing Cr oxide foams or reticulated ceramics. The active catalyst structures, catalyst supports and syngas production methods of the present invention are able to overcome some of the shortcomings of previous catalysts and syngas production processes by permitting short contact time of the reactant gases with the catalyst bed, and allowing high flow rates of reactant and product gases. Due to the favorable structure of the catalyst, or catalyst support, this is accomplished without causing excessive back pressure. The new catalysts and supports resist deterioration under operational temperature and pressure conditions better than typical catalysts in use today for syngas production. The preferred new catalysts comprise reticulated ceramic structures, preferably ceramic foams, and demonstrate excellent levels of conversion of methane and oxygen reactants and selectivities for CO and H2 products by a predominantly, or net partial oxidation reaction.
According to certain embodiments, the reticulated foam catalyst contains one or more metal oxides of chromium, cobalt, nickel, an alkaline earth, a rare earth, or another sinterable metal oxide that is active in any of various chemical oxidation reactions, preferably the catalytic partial oxidation of methane to synthesis gas.
In certain preferred embodiments of the catalysts of the present invention, chromium-containing macroporous three-dimensional structures, or monoliths are provided which are self-supporting chromium-containing catalysts made of reticulated ceramic materials or three-dimensional ceramic foams.
Preferred methods of making the chromium oxide-containing catalysts include processing a chromium oxide powder in a reducing atmosphere to avoid production and evolution of volatile chromium compounds (e.g., CrO3) during heating.
In some embodiments of the new catalysts, catalyst structures comprising a higher surface area active catalyst phase formed on top of a preformed reticulated ceramic foam are provided.
In other embodiments of the new catalysts, less active or non-catalytically active monoliths comprising reticulated ceramic materials, or three-dimensional ceramic foams, containing one or more metal oxides of chromium, aluminum, zirconium, titanium, magnesium, cobalt, nickel and silicon are provided. These structures serve as non-poisoning supports for active or more active catalyst materials for various oxidations, including methane oxidation to synthesis gas. Another aspect of the present invention is a process or method of making the above-described catalysts and support structures.
Yet another aspect of the invention comprises a process or method of converting a gaseous methane or other light hydrocarbon feedstock, together with an oxygen source and, optionally, nitrogen, catalyzed by one of the above-described catalysts, to yield a product containing a mixture of carbon monoxide and hydrogen gases. Still other embodiments and advantages of the present invention will appear from the following description.